Receptor tyrosine kinases (RTKs) mediate intercellular signals essential for the development and maintenance of the cells of multicellular organisms. The minimal domain structure of RTKs consists of an extracellular ligand-binding domain, a single transmembrane helix and a cytoplasmic kinase domain. This minimal structure, however, is very rare and typically the extracellular moiety of RTKs, the ectodomain, consists of complex and distinctive domain sets which enable classification of the RTKs in different families (1).
There is a strong preference for certain domains to occur in the ectodomain of RTKs. The fibronectin type-3 (FN-3) domain, for example, is present as 2 copies in the large Eph receptor family, 3 copies in the insulin and IGF-1 receptors and at least 7 copies in the ROS receptor (1). Cysteine-rich domains of variable length are also commonly found in RTKs.
A large number of RTKs contain immunoglobulin (IG) domains and the ectodomain of certain families consists solely of IG domains: the FGF receptors contain 2 or 3, depending on RNA splicing, the PDGF, CSF1, KIT and FLK2/STK1 receptors contain 5 and the FLT1, FLK1, FLT4 and CCK4 receptors contain 7 (1). IG domains can also be present in combination with FN-3, cysteine-rich or other domains (1). Interestingly, most IG domains present in RTKs and cell adhesion molecules belong to a distinct structural set known as the ‘I set’, with architecture intermediate between the V and C1 sets (2).
MET, the RTK encoded by the c-met proto-oncogene (3, 4), is the receptor for HGF/SF (5) a large polypeptide growth factor discovered as a protein causing dispersion of epithelial colonies and cell migration (scatter factor) (6, 7) and as a liver mitogen (hepatocyte growth factor) (8-10). HGF/SF and MET are essential for the development of several tissues and organs including, the placenta (11, 12), liver (11), and several groups of skeletal muscle (13). They also play a major role in the abnormal migration of cancer cells as a result of over-expression or MET mutations (14). In contrast to extensive data on the signal transduction pathways activated by MET (15), little is known about extracellular MET.
The involvement of MET in the spread of tumours makes this gene a suitable target for the development of antagonists which might prevent the activation of this RTK. The development of suitable assays involving large complex proteins can be difficult, particularly where is it desired to have a robust process suitable for high-throughput screens. This can be particularly problematical where, as with MET, receptor dimerization is believed to be required for binding to its cognate ligand.
Mark et al, J. Biol. Chem. 1992, 267; 26166-26171, describe fusions of the extracellular domain of the MET receptor to the constant region of an IgG heavy chain. These fusions produce soluble met protein which forms a dimer through the presence of the heavy chain region.